Analog-to-digital converters (ADCs) of many kinds have been used with various kinds of measurement systems. For examples, ADCs have been employed in sensors and transducers for thermocouple and piezoelectric sensor applications. Typically, such systems produce no voltage output signals at all, unless they are provided with a specific input such as heat or pressure. Input signals from such sensor applications are thus inherently ground-based. In other words, the input signals from the sensors express value levels with reference to a zero or ground potential. The signal input relative to ground may be positive or negative.
Performing analog-to-digital signal conversions on analog signals going below ground levels is particularly difficult with ADCs which run on 0V and 5V supplies only. A significant problem of these ADC systems is level shifting the negative and positive voltages produced by particular sensors and transducers when interfaced with single supply ADCs. Level shifting of low noise, high accuracy signals is expensive and complicated. In the past, only dual supply converters with positive and negative 5V supplies could convert negative analog voltage signals. In particular, measurement systems with analog front end circuits and ADCs have relied upon dual supply voltages. With dual supply voltages, circuits could be designed with components which could utilize both positive and negative power supply voltages. The power supply voltage levels at one time ranged from positive 15 to negative 15 volts. More recently, the range of voltages serving as power sources for semiconductor chips has diminished to positive 5 and negative 5 volts, or even, in some instances, to as low as 3 volts.
To protect sensitive analog circuitry on monolithic ADCs, large diodes have been connected to the input pin leads associated with the first and second ADC power supplies. In the case of a single supply being used as the semiconductor chip power source, the ADC diodes are connected respectively to ground potential and to VDD in order to protect the on-chip ADC circuitry. This protection is effective, because if the semiconductor chip input signal pin goes above or below the level of either of the predetermined supply voltages, the corresponding diode turns on. For example, if the input pin of the ADC is driven above the level of VDD or below the level of ground, a corresponding diode turns on and clamps the associated voltage at the VDD or ground level, as applicable. The diodes as configured accordingly are effective to protect the sensitive internal circuitry of the ADC. Since the turn-on voltage of each diode is approximately 0.6 volts in each case, an input pin to the ADC can in principle operably be taken below the level of ground. However, the diode turn-on action is not instantaneous, but subject to a non-linear exponential turn-on transfer curve. The nonlinearity of the transfer curve detrimentally affects ADC performance. However, without the diodes, the input pin becomes subject to damage from electrostatic discharge (ESD) events.
FIG. 1 is a diagram of a monolithic analog-to-digital converter (ADC) 7 according to the prior art. ADC 7 is conventionally connected impedance resistor 8, a source voltage 9, and a ground terminal 10. ADC 7 includes internal circuitry 7A, and first and second diodes respectively 7B and 7C, also respectively referred to as diodes D1 and D2. ADC 7 further includes respective first and second voltage connections 7D and 7E, respectively VDD and ground (GND), and an analog input connection 7F. Voltage connections 7D and 7E are according to one embodiment respectively connected to five volts (5V) and zero volts (0V). Diodes D1 and D2 are reverse biased and connected in series between VDD and ground, with the anode of diode D1 being connected to the cathode of diode D2. Further, the anode of diode D2 is connected to ground, and the cathode of diode D1 is connected to VDD. Source impedance resistor 8 is connected between analog input connection 7F and source voltage 9. Source voltage 9 is connected between ground 10 and source impedance 8. Diodes D1 and D2 are large in current carrying capacity to protect the sensitive analog circuitry of the monolithic ADC 7. In the case of a single supply, the diodes to both ground and VDD protect the on chip circuitry by forward biasing, if the input is driven above VDD or below ground. For example, if analog input connection or pin 7F is driven above VDD or below ground, one of the diodes turns on and clamps the pin 7F.
FIG. 2 is a current voltage characteristic curve diagram for a diode according to the prior art. As a result of this characteristic, a diode does not instantly turn on. When a particular diode is forward biased more than approximately 0.6 volts, it begins to conduct current. However, the turn-on current is highly non-linear.
It is accordingly desirable to develop an ADC circuit design which accommodates a range of input signal levels, including both positive and negative levels. Moreover, it is desired to create an ADC circuit which operates in a substantially linear mode, even when acting upon input analog voltage levels which traverse ground by having components that are positive as well as components which are negative in polarity. Further, it is desired that the ADC circuit design be effective for minimizing noise and increasing the signal-to-noise ratio of the overall circuit design.